Modulation of laser beam power by means external to the laser cavity is advantageous for maintaining stability of the wavelength and may provide much higher modulation rates and more accurate power selection in comparison to intracavity means. Several modulator systems using waveguide gratings and electrooptic effects are known. Selection of a system depends on the desired characteristics of the modulator, including speed and modulation of power amplitude, and the ease of inserting the device in an optical system.
The principle of modulation using guided-mode resonance properties of waveguide gratings (i.e., the phenomenon of coupling from a radiative mode to a guided mode), is disclosed by R. Magnusson and S. Wang in "New principles for optical filters," Appl. Phys. Lett., 61, 1022-1024 (1992). Several potential applications for this type of modulator me suggested including: special filters (static and tunable), low power switching units, line-narrowing reflectors for laser cavities and possibly integrated-optics. A modeled system is used to demonstrate the example of the wavelength sensitivity of coupling resonance. The authors indicate that the width of the response curve can be tuned by controlling the depth of the grating grooves which in turn increases the attenuation coefficient of the grating coupler.
A light modulator which uses coupling resonance where an electro-optic material is deposited on the grating surface and supports a mode of propagation is disclosed by R. Reinish et al. in "Fast Pockels light modulators using guided wave resonance," Appl. Optics, 24, 2001-2004, (1985). The system was modeled for a change in the refractive index resulting from adjusting the applied voltage while all other parameters were held constant. The dependence of the input coupling efficiency on applied voltage was observed to be Lorentzian and the width of the response increased with a thinner waveguide layer or an increase in the mode of the guided beam. Again, the attenuation coefficient of the grating coupler is increased by changing these parameters.
Bragg modulation using the electro-optic effect also has been disclosed by Arhmedzhanov et al. in "Linear thin-film Bragg modulator," Elect. Lett., 20, 967-968 (1984). However, this method does not utilize the coupling resonance between radiative and guided modes; rather, the optical beam modulated is confined within the waveguide.